Field of the Invention
The invention lies in the field of semiconductors. The present invention relates to power transistor cells, particularly heterojunction bipolar transistor power cells or monolithic microwave integrated circuits (xe2x80x9cMMICxe2x80x9d) power cells, which are thermally optimized by air bridges.
Heterojunction bipolar transistors (xe2x80x9cHBTxe2x80x9d) are required for many applications using the microwave range, e.g., for mobile communication techniques. The particular advantages of HBTs and HBT-MMICs include high current-carrying capacity at high current densities, the high power amplification achieved with the current capacity and density, and the correspondingly high efficiency. These advantages can frequently not be achieved due to poor thermal conductivity of the semiconductor (e.g., gallium arsenide (GaAs)) substrate material. It is, therefore, necessary to ensure sufficient dissipation of the heat produced in the cells. Up to now, various means have been used in order to ensure sufficient heat dissipation. For example, the substrates used can be thinned to a thickness corresponding to the spacing between the heat sources. Metal-filled wells etched into the back of the substrate can be used as heat sinks below the transistors. Gold-plated holes can be used from the back of the substrate to the connection contact surfaces on the top in order to dissipate heat. The transistors can be mounted using the flip-chip construction technique. It is also possible to mount the transistors from the emitter connection surfaces on heat sinks using bumps, and to connect the collector and base regions on the back of the substrate. Appropriate holes are present for the electrical connection. It is also possible to grow the semiconductor layers of the transistor epitaxially on a highly thermally conductive substrate. Use of emitter or base ballast resistors can reduce the adverse effects of heat evolvement on the current stability of the transistor, but such use compromises the radio-frequency performance of the transistor.
Using air bridges provides one possibility for direct heat dissipation through the electrical connections of the transistor regions. Such air bridges are made of electrically and thermally conductive material, preferably of a suitable metal, and connect the contacts of the individual transistors in a cell electrically and thermally to one another in the form of a bridge. Such air bridges are described in the following publications: L. L. Liou et al., xe2x80x9cThe Effect of Thermal Shunt on the Current Instability of Multiple-Emitter-Finger Heterojunction Bipolar Transistorsxe2x80x9d, IEEE 1993 Bipolar Circuits and Technology Meeting, pp. 253-256, 1993; B. Bayraktaroglu et al., xe2x80x9cVery High-Power-Density CW Operation of GaAs/AlGaAs Microwave Heterojunction Bipolar Transistorsxe2x80x9d, IEEE Electron Device Letters 14, 493-495 (1993); T. Miura et al., xe2x80x9cHigh Efficiency AlGaAs/GaAs Power HBTs at a Low Supply Voltage for Digital Cellular Phonesxe2x80x9d, GaAs IC Symposium 1996 Dig., pp. 91-94, 1996; R. Anholt et al., xe2x80x9cDecoupled Electrical/Thermal Modeling of AlGaAs/GaAs Heterojunction Bipolar Transistorsxe2x80x9d, IEEE GaAs IC Symposium Dig. 1996, pp. 167-170, 1996. With these HBTs, the emitter fingers are connected to one another using a respective air bridge. Connection of the collector metal connections of the individual HBT subcells through air bridges is described, for example, in the publication by H. F. Chau et al., titled xe2x80x9cHigh-Power, High-Efficiency K-Band AlGaAs/GaAs Heterojunction Bipolar Transistorsxe2x80x9d, IEEE GaAs IC Symposium 1996 Dig., pp. 95-98, 1996. A plurality of the measures described can be combined.
It is accordingly an object of the invention to provide a power transistor cell that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that is improved further with respect to thermal optimization.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a power transistor cell, including an air bridge, and a plurality of individual transistors each having at least one separate connection contact, each of the at least one separate connection contact being thermally conductively connected to one another through the air bridge forming air bridge connections, the air bridge connections defining a contact plane, a surface of the contact plane that contains each connection path between two of the air bridge connections defining a convex region, the air bridge formed to have, in the contact plane, dimensions that exceed a smallest convex region containing all of the air bridge connections in all directions of the air bridge.
With the power transistor cell according to the invention, an air bridge is used that has a considerable extent not only in the direction of a series of individual transistors oriented in a row, but that also has a considerable extent perpendicularly to such direction in the plane of the substrate surface. Such an air bridge can, firstly, be produced to be particularly thick and, therefore, highly thermally conductive, and, secondly, can be provided with good heat dissipation or a heat sink, using additional thermal contacts on connection contact surfaces that are dimensioned to be relatively large or on the substrate surface. In the publications indicated, air bridges are described that are used to connect the emitter fingers, i.e., the finger-shaped emitter contacts, to one another using air bridges whose width is no larger than the length of the emitter fingers. With the normally short emitter lengths of between 20 xcexcm and 30 xcexcm, the transportation of heat in such essentially one-dimensional air bridges is effective in only one direction along the bridge. The relatively small lateral extent of the air bridge affords a correspondingly small volume to take up the heat produced (low thermal capacitance); the total heat dissipation to the chip surface occurs only through the two outermost transistor cells. In contrast to this, the air bridge present in the power transistor cell according to the invention has as large a surface as possible and, therefore, has a high thermal capacitance. By virtue of the air bridge also having a considerable extent in the longitudinal direction of the emitter fingers, it is also possible to support the air bridge by thermally conductive contact pillars or supports mounted on the chip top or on contact surfaces and, at the same time, to dissipate the heat into the substrate or into metal layers applied thereon. It is also possible to route such thermally conductive connections through via holes in the substrate onto the back of the substrate. The air bridge present in the power transistor cell according to the invention is called a two-dimensional air bridge below, in order to distinguish it from the air bridges described in previous publications.
In accordance with another feature of the invention, the at least one separate connection contact is elongate and has a length, and the air bridge protrudes beyond the at least one separate connection contact by at least 15% of the length of the at least one separate connection contact in a longitudinal direction of the at least one separate connection contact.
In accordance with a further feature of the invention, the at least one separate connection contact is elongate and has a length, and the air bridge protrudes beyond the at least one separate connection contact by at least 30% of the length of the at least one separate connection contact in a longitudinal direction of the at least one separate connection contact.
In accordance with an added feature of the invention, the at least one separate connection contact is elongate and has a length, and the air bridge protrudes beyond the at least one separate connection contact by at least 100% of the length of the at least one separate connection contact in a longitudinal direction of the at least one separate connection contact.
In accordance with an additional feature of the invention, the air bridge has a surface and has a thickness perpendicular to the surface of between 20 xcexcm and 30 xcexcm.
In accordance with yet another feature of the invention, there is provided a chip with a chip top, and a plurality of thermally conductive contact pillars, the air bridge being mounted on the chip top by the plurality of thermally conductive contact pillars.
In accordance with yet a further feature of the invention, there is provided a plurality of thermally conductive contact pillars, the air bridge mounted on the convex region by the plurality of thermally conductive contact pillars.
In accordance with yet an added feature of the invention, the air bridge and the plurality of thermally conductive contact pillars are electrically conductive, and the at least one separate connection contact is electrically conductively connected to the convex region by the air bridge and the plurality of thermally conductive contact pillars.
In accordance with yet an additional feature of the invention, the plurality of individual transistors is a plurality of bipolar transistors each having an emitter, a base, a collector, the at least one separate connection contact is one of group consisting of at least one emitter finger formed as an emitter connection contact, at least one base finger formed as a base connection contact, and at least one collector contact formed as a collector connection contact, and every one of the at least one separate connection contact is thermally conductively connected to one another by the air bridge.
In accordance with again another feature of the invention, there is provided one of a group consisting of an emitter connection surface, a base connection surface, and a collector connection surface, and a plurality of thermally and electrically conductive contact pillars, the air bridge being electrically conductive and mounted on a respective one of a group consisting of the emitter connection surface, the base connection surface, and the collector connection surface by the plurality of thermally and electrically conductive contact pillars.
In accordance with again a further feature of the invention, there is provided a base connection surface, the at least one separate connection contact being at least one base finger formed as a base connection contact, and the air bridge being mounted on each of the at least one base finger and on the base connection surface.
In accordance with again an added feature of the invention, the at least one base finger is at most 1 xcexcm wide.
In accordance with again an additional feature of the invention, there is provided a heat sink, the air bridge being an integral component of the heat sink.
In accordance with still another feature of the invention, there is provided a housing, the air bridge being an integral component of the housing.
In accordance with a concomitant feature of the invention, there is also provided a block of power transistor cells, including an air bridge, and a plurality of power transistor cells, each of the a plurality of power transistor cells including a plurality of individual transistors each having at least one separate connection contact, each of the at least one separate connection contact being thermally conductively connected to one another through the air bridge forming air bridge connections, the air bridge connections defining a contact plane, a surface of the contact plane that contains each connection path between two of the air bridge connections defining a convex region, the air bridge formed to have, in the contact plane, dimensions that exceed a smallest convex region containing all of the air bridge connections in all directions of the air bridge, whereby each of the plurality of power transistor cells are respectively thermally conductively connected to one another through the air bridge to form a block of power transistor cells.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a power transistor cell, it is nevertheless not intended to be limited to the details shown, because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.